The rate of pressure buildup of gas emanating from an automobile air bag inflator is subject to strict requirements to avoid injury to out of position vehicle occupants. Aggressive, uncontrolled onset rates of pressurization are undesirable since excessively rapid inflation of the air bag may impact against the occupant. Essential to the concept of the present invention is the discovery that crushing of the propellant grains adjacent to the initiator of the inflator due to a shock wave developed by the initiator is a contributory cause of such excessive onset rates.